1. Field of the Invention
The present invention relates to a rare earth sintered magnet with improved corrosion resistance.
2. Description of the Related Art
A rare earth permanent magnet having an R-T-B (R is a rare earth element and T is one or more transition metal elements including Fe or Fe and Co) composition is a permanent magnet that has a structure including a main phase containing an R2T14B phase of a composition formula of R2T14B and a grain boundary phase containing an R-rich phase in which the content of R is larger than that of R2T14B. Such a rare earth magnet exerts excellent magnetic properties such as a high coercive force HcJ. An R-T-B rare earth permanent magnet is used as a high performance permanent magnet in motors and the like particularly requiring high performance, such as voice coil motors (VCM) for driving hard disk drive (HDD) heads, electric cars, and hybrid cars.
A rare earth permanent magnet contains R in the composition thereof and thus has high activity. However, R is easily oxidized to have low corrosion resistance, and therefore, various studies are conducted for improving the corrosion resistance. Typically, the surface of a rare earth magnet is plated with nickel (Ni) or other materials to increase corrosion resistance.
Improvement of the corrosion resistance of a rare earth permanent magnet itself is extremely important for making a rare earth magnet coated by plating or other methods be more reliable. Studied is improvement of the corrosion resistance of a rare earth magnet by typically adding an element such as Co and Cu as an element for improving the corrosion resistance.
Conventionally, for example, Japanese Laid-open Patent Publication No. 2003-31409 discloses a rare earth sintered magnet in which an intermediate phase containing Co and Cu of an atomic weight ratio of 30% to 60% is formed around an R-rich phase being present in a grain boundary triple point where a plurality of grain boundaries are merged. Thus, R in the R-rich phase in the grain boundary triple point is suppressed from being oxidized to improve the corrosion resistance.
However, the progress of corrosion cannot be sufficiently suppressed by simply covering the periphery of the R-rich phase being present in the grain boundary triple point with the intermediate phase containing Co and Cu because the grain boundary triple point includes a high proportion of the R-rich phase.
In other words, oxidation of R is suppressed from progressing toward the inside of the grain boundary phase by covering the periphery of the R-rich phase with the intermediate phase in the grain boundary triple point. However, when pinholes or the like occur in the region of the triple point on the surface of the magnet, oxidation of R cannot be sufficiently suppressed by simply covering the R-rich phase with the intermediate phase in the grain boundary triple point because the grain boundary triple point includes a high proportion of the R-rich phase. As a result, the oxidation of R may not be suppressed from progressing toward the inside of the grain boundary phase.
In recent years, rare earth sintered magnets have been increasingly used in automobiles, industrial equipment, or the like. Therefore, rare earth sintered magnets excellent in corrosion resistance are required in order to provide rare earth sintered magnets also available for such applications more stably.